1. Field of the Invention
The present invention relates to a bi-directional transmitter/receiver for time division transmission of a signal in two ways between first and second transmitter/receivers, the signal being modulated using a spread spectrum method.
2. Description of the Related Art
Generally, a bi-directional transmitter/receiver system for time division bi-directional communication has been known. Referring to FIG. 1, a first transmitter/receiver transmits signals to a second transmitter/receiver using a predetermined RF frequency during a predetermined period, and the second transmitter/receiver then transmits signals to the first transmitter/receiver using the same RF frequency during a predetermined period. This is refereed to as a Time Division Duplexer, or TDD, communication method. Communication between the first and second transmitter/receivers is made through repetitive time division transmissions between them. As one example of such communication systems, there is a system which employs a spread spectrum method as a modulation/demodulation method. On the transmitter side, for example, audio data or the like is subjected to spread spectrum modulation by using a spread code, and then frequency conversion into an RF frequency. On the receiver side, the received spread spectrum modulation signal is subjected to spread spectrum demodulation by using a spread code to obtain audio data or the like.
With the spread spectrum method, audio data or the like cannot be demodulated unless a spread code in the received spread spectrum modulation signal and a spread code generated by the receiver are synchronized with each other because demodulation cannot properly be achieved unless the two spread codes are synchronized. With this in mind, synchronization is ensured between the two spread signals by using a PLL method as disclosed in JPA Hei 8-265214.
To ensure synchronization of two spread codes on the receiver side, VCO oscillation frequencies for PLL are forcibly varied from higher to lower frequencies until synchronization is detected. When synchronization is detected forcible variation is halted. This is referred to as sliding correlation, which has an advantage of smaller power consumption. The sliding correlation, therefore, is advantageous in view of power consumption when at least one of the transmitter and the receiver of the transmission/receiving system shown in FIG. 1 is a portable phone (e.g., a handset of a cordless telephone device).
However, it takes time until two spread codes are synchronized when the sliding correlation in which oscillation frequencies of VCO are varied is used as it is not known when synchronization is established. As it is, all transmission data cannot be demodulated with only the employment of a sliding correlation device as time is largely consumed for establishment of synchronization within one receiving period. Therefore, a spread spectrum method using a sliding correlation device is not favorably used in a time division bi-directional communication system.
According to one aspect of the present invention (claim 1), there is provided a bidirectional transmission/receiving system for time division communicating a signal in two directions between first and second transmitter/receivers, the signal being modulated using a spread spectrum method. In this system, one of the first and second transmitter/receivers sending data includes a pilot carrier for making the other of the first and second transmitter/receivers establish synchronization for spread spectrum signals, and a communication carrier including an information data.
Also, in the above system, the pilot carrier is generated by carrying out spread spectrum on data with a predetermined pattern.
Further, in the above system, one of the first and second transmitter/receivers establishes synchronization for the spread spectrum signals based on the pilot carrier, and then demodulates the information data included in the communication carrier through spread spectrum.
Still further, in the above system, the first and second transmitter/receivers each establish synchronization between a received spread spectrum signal and its own generated spread signal through sliding correlation during a pilot carrier period.
According to another aspect of the present invention, there is provided a transmitter/receiver for time division communicating a signal in two directions with a paired transmitter/receiver, the signal being modulated in a spread spectrum method. This transmitter/receiver sends data to the other paired transmitter/receiver, the data including a pilot carrier for making the paired transmitter/receiver establish synchronization of a spread spectrum signal, and a communication carrier including information data.
Also, in the above transmitter/receiver, the pilot carrier is dummy data generated by giving spread spectrum to data with a predetermined pattern.
Further, when a spread spectrum signal is received from the other paired transmitter/receiver, the above transmitter/receiver establishes synchronization for a received spread spectrum signals based on the pilot carrier, and then demodulates the information data included in the communication carrier through despread spectrum.
According to the present invention, data for transmission includes a pilot carrier for use in establishment of spread spectrum signal synchronization, and a communication carrier including information data. With this arrangement, the transmission data can be accurately demodulated for reproduction with the spread spectrum signals well synchronized.
That is, the present invention can advantageously achieve favorable time division bi-directional communication even when a spread spectrum demodulator is used, which synchronizes received and self-generated spread codes through sliding correlation and thus takes time to establish the synchronization.
Still further, in the above system, the first and second transmitter/receivers shift from being in an initial synchronization state in which the first and second transmitter/receivers are not synchronized, to being in a normal receiving state in which the first and second transmitter/receivers communicate spread spectrum signals. The data which one of the first and second transmitter/receivers in the initial synchronization state and the normal receiving state sends to the other of the first and second transmitter/receivers includes the pilot carrier and the communication carrier. The pilot carrier is kept being sent in the initial synchronization state for a longer time than in the normal receiving state.
Still further, the transmitter/receiver shifts from being in an initial synchronization state in which the transmitter/receiver is not synchronized with the paired transmitter/receiver, to being in a normal receiving state in which the transmitter/receiver communicates spread spectrum signals with the paired transmitter/receiver. The data which the transmitter/receiver sends to the other of the paired transmitter/receivers in the initial synchronization state and the normal receiving state includes the pilot carrier and the communication carrier. The pilot carrier is kept being sent in the initial synchronization state for a longer time than in the normal receiving state.
That is, it takes a longer time to establish synchronization in the initial synchronization state than in a normal receiving state because the first and second transmitter/receivers in the initial synchronization state are totally out of synchronization. Thus, a pilot carrier period is set longer in the initial synchronization state than in a normal receiving state so that the first and second transmitter/receiver which are not synchronized with each other at all can more reliably establish synchronization, and the following communication carrier can therefore be more accurately demodulated.
Further, in the above system, the first and second transmitter/receivers each have a number of channels usable for communication, and at least one of the first and second transmitter/receivers detects, through a search, an empty channel from among the number of channels while being in a data transmission/receiving mode so that the empty channel detected is used for data transmission/receiving.
When an empty channel is detected through a search, communication can automatically be made via an empty channel. Therefore, even when, for example, communications using different communication methods from the spread spectrum method are carried out in a frequency band where the communication using a spread spectrum method is carried out, the communication using a spread spectrum method is not hindered by the other communication so that data can be easily and accurately communicated between the first and second transmitter/receivers.
Still further, in the above system, one of the first and second transmitter/receivers starts counting a predetermined time upon starting sending data to the other of the first and second transmitter/receivers, and the other of the first and second transmitter/receivers starts counting a time of the same length upon starting to receive the data. The first and second transmitter/receivers both reverse transmission/receiving states after the predetermined time has elapsed.
When time periods of the same length are counted automatically at every start of data transmission/receiving so that data transmission/receiving states are reversed after the predetermined time period has elapsed, time can be easily and acurately managed among respective transmitter/receivers.